Porous media burner for low calorific value fuel gases

ABSTRACT

A method and burner for combusting fuels having a heating value of &lt;80 BTU/scf (&lt;2.98 MJ/Nm 3 ) in which the fuel premixed with air is injected into a porous inert media so that the fuel is combusted with the air. Oxygen may be optionally injected into the porous inert media separately from the premixed air/fuel.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

Background

1. Field of the Invention

The present invention relates to combustion of fuels having lowcalorific values of <50 BTU/scf (<1.863 MJ/Nm³) that are derived fromreject streams from landfill gas or digester gas purification processes.

2. Related Art

Traditionally, fossil fuels such as natural gas, coal and petroleum havebeen used in industrial furnaces and boilers as energy sources. Severalwell-established combustion technologies (whether it is combusted withoxygen or air) are commercially available for burning such fuels. Insome industrial processes, large amounts of by-product gases aregenerated that contain calorific values typically lower than thetraditional fuels. While most of the by-product gases are utilized inthe same process or plant, a significant portion is still flared withoutany energy recovery. Low calorific value (LCV) fuels having lowerheating values (LHV) of less than 188 BTU/Nft³ (less than 7 MJ/Nm³) areoften available from various industrial processes such as landfills orbio-gas plants. Reed, et al. reported typical LHVs for various LCV fuelswhich are listed below in Table I (Reed, R. J. “Combustion Handbook”,North American Mfg. Co., Third Edition, Vol. 1, 2001.).

TABLE I Composition and lower heating values of LCV fuels CompositionLHV Gas CH₄ C₂H₆ C₃H₈ CO H₂ CO₂ O₂ N₂ H₂O BTU/scf MJ/Nm³ BFG — — — 22.72.3 19.3 0.7 — —  80 2.98 COG 28.3 3.4 0.2 4.2 50.6 0.9 1.6 — — 475 17.7LG 45-65 — — 0-1 0-1 34-55 0-5 0-1 sat. 240-550 9-20 BGP 1.84 — — 2.1 2112 — 43 — 190 7.07 BFG: blast furnace gas COG: coke oven gas LG:landfill gas BGP: Batelle gasification process sat.: saturatedOne of ordinary skill in the art will recognize that the above LHVvalues may be converted to Wobbe Index values by the well knownequation:

${{Wobbe}\mspace{14mu} {Index}} = \frac{LHV}{\left. \sqrt{}\rho_{re} \right.}$

Due to the presence of large amounts of diluents in the fuelcomposition, LCV fuels present certain combustion challenges. Thesefuels are prone to poor ignition and low flame stability. They are alsosusceptible to flame extinction or blow-off. In an effort to improveflammability, these fuels are sometimes blended with conventional fuels(called “sweetening”). These fuels may also be swirled, injected in acyclone burner and/or preheated.

The flammability limits are characteristics of the fuel and depend onmixture composition and temperature. Determination of flammabilitylimits depends on measurement of residence time and heat loss.Flammability limits of mixtures of fuels and diluents are estimatedusing le Chatlier rule as given below:

$x_{fm} = \frac{100}{\frac{p\; 1}{x_{f\; 1}} + \frac{p\; 2}{x_{f\; 2}} + \frac{p\; 3}{x_{f\; 2}}}$

where:

xfm: the flammability of the mixture in vol %

xfi: the flammability of the submixture of component i with a diluents

pi: the vol % of the submixture of component i in the mixture

Thus, it is seen that a combination of lower flammability and higher vol% of one of the components can substantially lower the flammability ofthe mixture.

Fuels with LHV <50 BTU/Scf (<1.863 MJ/Nm³)—also called as ultra low BTUor ULBTU fuels—are extremely difficult to satisfactorily burn them withconventional burners. ULBTU fuel, which is often a reject stream from anindustrial process, is therefore typically flared by blending it with aconventional fuel. One example of such a reject stream is disclosed byU.S. Pat. No. 7,025,803. After pre-treatment via filtration andadsorption, a landfill gas is separated into CO₂-rich and CO₂-deficientstreams by one or more gas separation membrane stages. The CO₂-richstream may or may not be used to regenerate a PSA adsorbent bed. Becauseof the difficulty of combusting the CO₂-rich stream (with or withoutbeing used as a regeneration gas for a PSA adsorbent bed), that streamis instead treated in a thermal oxidizer.

Catalytic combustors can be used to burn ULBTU fuels, however, catalystdegeneration, poisoning, and regeneration is a major concern. Thermaloxidation is a technology that can be used to oxidize ULBTU fuels, butthermal oxidizer units are complex and expensive.

Several burners have been developed that can be used with LCV fuels.Niska, et al. of MEFOS and Linde developed a blast furnace gas oxy-fuelburner for steel reheating furnaces. This burner (the “S3 burner”) isbased on the REBOX® flameless technology and uses an optional boosterfuel. While the burner yielded lower NOx and CO emissions, the authorsconcluded that, for high productivity, propane boosting should beemployed.

U.S. Pat. No. 7,448,218, U.S. Pat. No. 5,433,600, U.S. Pat. No.5,447,427, and US 2004/0175663 A1 disclose other examples of burnersdeveloped for LCV fuels.

Catalytic combustors are typically employed for combustion of ULBTUfuels. For example, U.S. Pat. No. 6,393,821 discloses a process in whichgaseous fuel from natural evolution associated with rotting of materials(e.g. landfills, gas digesters, livestock refuse) is mixed with air,compressed, pre-heated, and catalytically combusted. The combustionproducts are directed to a turbine to produce electricity. Methaneconcentrations as low as 1% by volume can be combusted in a catalyticcombustor. As discussed above, catalyst degeneration/poisoning andregeneration are major concerns.

Similarly, US 2007/0098604 discloses a catalytic reactor for reacting afuel rich mixture of a low-BTU fuel and air, without an excessivepressure drop. The reactor includes an assembly of catalyst-coated tubeshaving an exit area that is at least 50% larger than a close-packedassembly. A fuel-rich mixture of fuel and air is in contact with thecatalyst coating on the outside surface of the tubes. Cooling air flowsinside the tubes and gets pre-heated. The catalytically reacted fuel-airmixture meets with the preheated air at the exit of the tubes andachieves complete combustion. This combustor aids the combustion oflow-BTU fuels having lower flame temperatures by catalytically reactinga portion of the fuel and completing the rest of the combustionnon-catalytically. As discussed above, catalyst degeneration/poisoningand regeneration are major concerns.

Oxygen enhancement of LCV fuel combustion is less common. US 20110195366discloses a method for combustion of a low-grade fuel (a mixture oflow-grade and high-grade fuels with a LHV≦7.5 MJ/Nm³) using an existingair burner. The burner supplies the fuel from an opening that normallysupplies air and oxidant from an opening that normally supplies fuel.Both openings open out into a combustion zone downstream. The oxidant ispreferably 95% oxygen by weight. While combustion of LCV fuels having anLHV of less than or equal to 7.5 MJ/Nm³ are disclosed, there is nodisclosure of combustion of LCV fuels having an LHV of <80 BTU/scf(1.863 MJ/Nm³).

Thus, there is a need for methods and burners for combustion of fuelshaving low calorific values of <80 BTU/scf (<2.98 MJ/Nm³) withoutrequiring additional high calorific value fuels and without requiringcatalysts requiring replacement or regeneration.

SUMMARY

There is provided a burner for combusting fuels having a heating valueof <80 BTU/scf (<2.98 MJ/Nm³), comprising: an inner tube having openfirst and second ends; an outer tube concentrically disposed around theinner tube and having a closed first end and a second end, the inner andouter tubes defining annular space between them for allowing a flow ofpremixed air/fuel therethrough; and a porous inert media comprisingmetal or ceramic and having first and second ends that is disposedwithin the inner tube adjacent the inner tube first end. The annularspace has a second end that is closed by a face that extends between thesecond ends of the tubes. The face includes one or more openings forallowing a flow or flows of air and fuel. The outer tube first endextends past the inner tube first end.

There is also disclosed a method for combusting fuels having a heatingvalue of <80 BTU/scf (<2.98 MJ/Nm³) that includes the following steps.Air is mixed with a gaseous fuel having a heating value of <80 BTU/scf(<2.98 MJ/Nm³) to provide a flow of premixed air/fuel. The flow ofpremixed air/fuel is directed through an annular space defined by innerand outer tubes of a burner. The flow of premixed air/fuel is injectedinto a porous inert media disposed within the inner tube. The fuel iscombusted with the air to produce a flow of combustion products within adownstream portion of the inner tube in a direction opposite that of theflow of premixed air/fuel. Heat is exchanged between the flows ofcombustion products and premixed air/fuel across the inner tube. Theburner and/or method may include one or more of the following aspects:

-   -   an oxygen lance is included having first and second ends and        extending through a centrally disposed aperture formed in the        closed first end of the outer tube so that the lance second end        is disposed adjacent the first end of the media, wherein a space        is defined by the outer tube first end, an outer surface of the        lance, and an inner surface of the outer tube adjacent its first        end.    -   A screening material is disposed directly between the lance        second end and the media first end.    -   The lance second end abuts directly against the media first end.    -   A flow of oxygen is injected into the porous inert media        separate from the flow of premixed air/fuel for assisting the        combustion of the fuel with the air.    -   The fuel is a reject stream from a landfill gas or digester gas        purification process.    -   The oxygen is at least 95% pure oxygen.    -   A flame resulting from combustion of the fuel with the air and        oxygen is stabilized within the media.    -   A flame resulting from combustion of the fuel with the air and        oxygen is stabilized at a downstream face of the media.    -   The fuel has a heating value of <50 BTU/Nft³ (<1.86 MJ/Nm³).    -   The fuel is a reject stream from a landfill gas or digester gas        purification process.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is a cross-sectional elevation view of an exemplary burner foruse in the invention.

FIG. 2 is a top plan view of the burner of FIG. 1.

FIG. 3 is a cross-sectional elevation view of the burner of FIG. 1 takenalong line 3-3.

FIG. 4 is a cross-sectional elevation view of the burner of FIG. 1 takenalong line 4-4.

FIG. 5 is a cross-sectional elevation view of the burner of FIG. 1 takenalong line 5-5.

FIG. 6 is a bottom plan view of the burner of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

The solution proposed here is based on stabilizing a flame in or on aninert porous matrix created by combustion of LCV fuels having a LHV of<80 BTU/scf (<2.98 MJ/Nm³) or even as low as <50 BTU/Nft³ (<1.86MJ/Nm³). Unlike conventional burners, there is no free flame. Thepresence of the inert porous media establishes heat feedback from thehot combustion products so that the reactants may be preheated. Thisincreases the flame velocity, improves flame stability, and therefore,allows combustion of LCV gases that are harder to burn with traditionalburners. Oxygen may be optionally injected into the inert porous mediain order to further enhance the ignitability of the fuel and provide ahigher flame temperature in comparison to one produced from combustionof only the premixed fuel and air. While the air and fuel are premixed,for safety reasons the oxygen and the premixed air/fuel mixture areseparately supplied to the inert porous media.

The inert porous media is disposed within an upstream end of an innertube of the burner. In order to inject the optional oxygen separatelyfrom the air/fuel mixture, it is injected from a lance directly into anupstream face of the porous inert media. The open downstream end of thelance typically directly abuts against the upstream face. However, oneof ordinary skill in the art will recognize that the downstream end ofthe lance may be separated from the upstream face by a screen that isdesigned to support the media within the burner.

Different types of media are known in the field of porous media burners.Typically, the media is a metallic or ceramic foam. One example for suchmedia is open-cell silicon carbide coated carbon foam manufactured byUltramet (Pacoima, Calif.). Engineered foams made fromyttrium-stabilized zirconia can also be used for this purpose. There canbe one or more layers of porous media each having a same or differentporosity and number of pores per inch. The porous media layers can bestacked on top of one another or separated with a gap.

A combustion chamber is formed by the downstream end of the burner innertube. In other words, the combustion chamber is defined by thedownstream face of the inert porous media at one end, by the opendownstream end of the inner tube at the other end, and on all sides bythe downstream portion of the inner tube.

The combustion chamber of the burner is integrated with a flow path ofthe premixed air/fuel so that heat from combustion may be used topreheat the premixed air/fuel before it is introduced into the inertporous media. This is done in counter-flow fashion so that as thepremixed air/fuel flows in a direction opposite that of the combustionproducts. This is accomplished with the provision of an outer tubeconcentrically disposed around the inner tube. The downstream end of anannular space between the inner and outer tubes receives a flow of thepremixed air/fuel. The flow continues upstream through this annularspace, which is of course counter to the downstream direction of theflow of combustion products on the inside of the inner tube. The outertube extends farther upstream of the inner tube and has a closedupstream end. On the other hand, the upstream end of the inner tube isopen. Thus, the flow of premixed air/fuel exits the annular space,continues radially inward and then is introduced into the upstream faceof the inert porous media.

The fuel is then combusted with the air (of the air/fuel mixture) and,if selected, the optional oxygen (injected directly into the inertporous media from the open end of the oxygen lance). While the flame canbe stabilized on the surface of the porous inert media, greater heatexchange between the products of combustion and the premixed air/fuelmay be realized when the flame is stabilized within the media. Thus,combustion is initiated either within the media itself or within thecombustion chamber just downstream of the downstream face of the media.In order to stabilize the flame in the desired location, the pressure ofthe premixed air/fuel and/or the pressure of the optional oxygen may beincreased so as to increase the velocity of those gases through themedia.

The stability of the flame may also be enhanced by varying theallocation of oxidant between the air (of the premixed air/fuel) and theoxygen. Typically, as much as 5% of the oxidant bill may be supplied bythe oxygen with the balance supplied by the air in the premixedair/fuel. One of ordinary skill in the art will recognize that theamount of air within the premixed air/fuel may be varied by increasingor decreasing the pressure of the air to be mixed with the fuel, andoptionally conversely, by decreasing or increasing the pressure of thefuel to be mixed with the air. Such a one will further recognize thatdevices for mixing air and gaseous fuel are well known in the art andtheir details need not be repeated herein. In any case, no particulardevice for mixing the air and fuel is essential to the invention.Indeed, it is within the scope of the invention to separately feed airand fuel to the annular space and allow them to be mixed therein and/orinside the porous inert media. Typically, the air and fuel are mixed byinjecting or aspirating one of the two into a flow of the other of thetwo. The oxygen is industrially pure oxygen that may be sourced from anytype of oxygen production technology used in the industrial gasbusiness, including but not limited to:

-   -   an air separation unit that cryogenically separates air gases        into predominantly oxygen and nitrogen streams in which case the        gaseous oxygen has a concentration exceeding 99% vol/vol,    -   vaporization of liquid oxygen which was liquefied from oxygen        from an air separation unit, in which case it, too, has a purity        exceeding 99% vol/vol, or    -   produced by a vacuum swing adsorption (VSA) unit in which case        it typically has a purity of about 92-93% vol/vol.

The gaseous fuel has a LHV of <50 BTU/Nft³ (<1.863 MJ/Nm³). While theinvention is not limited to any particular source of fuel, examples ofsuch gaseous fuels include certain low calorific value blast furnacegases and off-gases from carbon black plants. One additional type ofgaseous fuel is a low-methane reject stream from a landfill gas ordigester gas purification process. These processes use one or more gaspurification techniques, such as gas separation membranes or pressureswing adsorption (PSA) that produce a low-methane content streamtypically having a high CO2 and/or N2 content . A particular example isthe reject stream disclosed in U.S. Pat. No. 7,025,803 that is otherwiseordinarily sent to a thermal oxidizer (i.e., stream 22). These fuelsshare a common attribute in that their calorific content is too low forseparating the high calorific components from the low calorificcomponents. It is important to note that the gaseous fuel does notinclude any supplementary or boosting fuel and that in practice of theinvention, no other fuel is combusted with the gaseous fuel in order tosupplement or boost its calorific value.

Raw low calorific value fuels typically contain significant amount ofimpurities (such as sulfur, H₂S, NH₃, particulates, etc.) which need tobe cleaned to some level before being utilized in energy recovery. Forinstance, blast furnace gas exits the furnace with a dust loading of 18to 34 g/m³ due to its passage through coke, iron ore and limestone. Asanother example, biogas typically contains ammonia and hydrogen sulfide.Well known techniques for gas cleaning include: i) low temperaturecleaning and ii) high temperature cleaning. In low temperature cleaningmethod, the raw LCV gas is typically cooled in a water-scrubber anddesulfurized to remove particulates and alkali metals and also to reducesulfur and ammonia concentration.

A typical embodiment of a burner for use in the invention (includinginjection of oxygen) will now be described.

As best illustrated in FIG. 1, the burner includes an inner tube 11concentrically surrounded by an outer tube 13 that define between theman annular space 15. The downstream end of the annular space 15 isclosed by a downstream face 17 of the burner. A premixed flow ofair/fuel is received into the annular space 15 via air/fuel inletconduit 19.

The outer tube 13 extends further upstream than does the inner tube 11so that an inner wall of the furthest upstream portion of the outer tubedefines a space 21. The upstream face 23 of the burner is also closed soas to direct the flow of air/fuel from the annular space 15 radiallyinward through the space 21. An oxygen lance 25 is disposed co-axialwith the inner and outer tubes 11, 13 and includes a downstream end 27that abuts against an upstream face 29 of a porous inert media 31. Aflame resulting from combustion of the fuel with the air and oxygenoxidants may be stabilized within the porous inert media itself 31 orstabilized on a downstream face 33 of the media 31. The products ofcombustion (N₂, CO₂, H₂O) flow in the upstream to downstream directionthrough the combustion chamber 35 and out of the burner.

In operation, the premixed air/fuel is introduced into the annular space15 via the conduit 19. Alternatively, separate conduits may separatelyfeed the fuel and the air into the annular space. The flow of premixedair/fuel within the annular space 15 flows in a direction opposite thatof the combustion products within the combustion chamber 35. The flow ofpremixed air/fuel is directed radially inwardly at the upstream face 23of the burner, through a space 21, and is introduced into the porousinert media 31 via its upstream face 29. A flow of oxygen is injectedinto a central portion of the inert porous media 31 directly from thedownstream end 27 of the lance 25. As discussed above, directly injecteddoes not exclude the possibility of including a screen or similarfeature in between the downstream end 27 and the upstream face 29. Thefuel is combusted with the air and oxygen oxidants with the flamestabilized either within the media 31 itself, or on the downstream face33 of the media 31. The products of combustion flow in an upstream todownstream direction within the combustion chamber 35 and out theburner.

Preferred processes and apparatus for practicing the present inventionhave been described. It will be understood and readily apparent to theskilled artisan that many changes and modifications may be made to theabove-described embodiments without departing from the spirit and thescope of the present invention. The foregoing is illustrative only andthat other embodiments of the integrated processes and apparatus may beemployed without departing from the true scope of the invention definedin the following claims.

What is claimed is:
 1. A burner for combusting fuels having a heatingvalue of <80 BTU/scf (<2.98 MJ/Nm³), comprising: an inner tube havingopen first and second ends; an outer tube concentrically disposed aroundthe inner tube and having a closed first end and a second end, the innerand outer tubes defining annular space between them for allowing a flowof premixed air/fuel therethrough, wherein: the annular space has asecond end that is closed by a face that extends between the second endsof the tubes, the face includes one or more openings for allowing a flowor flows of air and fuel, the outer tube first end extends past theinner tube first end; and a porous inert media comprising metal orceramic and having first and second ends that is disposed within theinner tube adjacent the inner tube first end.
 2. The burner of claim 1,further comprising an oxygen lance having first and second ends andextending through a centrally disposed aperture formed in the closedfirst end of the outer tube so that the lance second end is disposedadjacent the first end of the media, wherein a space is defined by theouter tube first end, an outer surface of the lance, and an innersurface of the outer tube adjacent its first end.
 3. The burner of claim1, further comprising a screening material disposed directly between thelance second end and the media first end.
 4. The burner of claim 1,wherein the lance second end abuts directly against the media first end.5. A method for combusting fuels having a heating value of <80 BTU/scf(<2.98 MJ/Nm³), comprising the steps of: mixing air with a gaseous fuelhaving a heating value of <80 BTU/scf (<2.98 MJ/Nm³) to provide a flowof premixed air/fuel; directing the flow of premixed air/fuel through anannular space defined by inner and outer tubes of a burner; injectingthe flow of premixed air/fuel into a porous inert media disposed withinthe inner tube; combusting the fuel with the air to produce a flow ofcombustion products within a downstream portion of the inner tube in adirection opposite that of the flow of premixed air/fuel; and exchangingheat between the flows of combustion products and premixed air/fuelacross the inner tube.
 6. The method of claim 5, further comprising thestep of injecting a flow of oxygen into the porous inert media separatefrom the flow of premixed air/fuel for assisting the combustion of thefuel with the air.
 7. The method of claim 6, wherein the fuel is areject stream from a landfill gas or digester gas purification process.8. The method of claim 5, wherein the fuel is a reject stream from alandfill gas or digester gas purification process.
 9. The method ofclaim 5, wherein the oxygen is at least 95% pure oxygen.
 10. The methodof claim 5, wherein a flame resulting from combustion of the fuel withthe air and oxygen is stabilized within the media.
 11. The method ofclaim 5, wherein a flame resulting from combustion of the fuel with theair and oxygen is stabilized at a downstream face of the media.
 12. Themethod of claim 5, wherein the fuel has a heating value of <50 BTU/Nft³(<1.86 MJ/Nm³).
 13. The method of claim 12, wherein the fuel is a rejectstream from a landfill gas or digester gas purification process.